When a projector and an observer of a projected image are located on the same side of a projection screen, the screen is commonly referred to as a front projection screen. Front projection screens typically include a support structure or substrate plus some overlaying coating which comprises a screen surface. The present invention is directed to features of the screen surface.
In a typical visual display system, the front projection screen, the projector, and the observer are positioned at different locations. This displacement of the observer's eye position from the location of the projector produces a condition referred to as off-axis viewing.
As discussed in U.S. patent application, Ser. No. 191,968 entitled Variable Gain Screen Permitting Off-Axis Viewing With Image Brightness Remaining Constant, by Dotsko et al., filed on Sept. 29, 1980, and assigned to the same assignee as the present invention, off-axis viewing causes the luminence or image brightness of the screen, as perceived by the observer, to change in correspondence with any change in viewing angle defined by the observer's eye line-of-sight. This Dotsko disclosure is incorporated by reference herein for a thorough exposition of the concepts just set forth.
The dependency of the perceived luminence of a screen, as a function of viewing angle, may be measured quantitatively and graphed. Perceived luminence is quantified by measuring the gain of the screen: the peak gain is defined as the measure of luminence or image brightness of the screen surface measured along a normal to the surface.
FIG. 1 shows a typical graph of gain versus viewing angle. In this figure, Curve A shows the relative luminence for a projection screen that uses a bright aluminum metal substrate coated with a lacquer film; Curve B shows a dull aluminum metal substrate coated with a lacquer film. These projection screens are described in further detail in U.S. Pat. No. 4,068,922 entitled "Front Projection Screen" which issued Jan. 17, 1978 to M. Dotsko.
It is observed in FIG. 1 that the peak gain quite rapidly "falls off" to a reduced "off-axis" gain as the viewing angle increases from zero degrees (where the gain is measured along the normal) to 90.degree.. For example, in Curve A, the gain is 100 at the normal but falls off to 10 to approximately 8.degree. off-axis. It is desirable, however, to have a screen surface that exhibits very slow gain fall off as the viewing angle increases, as this will provide the observer with constant image brightness for the whole screen surface.
The prior art has not adequately addressed the problem of rapid gain fall off as a function of viewing angle. For example, the above cited Dotsko application works around this problem by providing a screen surface that has a plurality of screen surface portions, each with a different peak gain. The peak gain varies from one surface portion to the next in such a manner as to compensate for the very rapid gain fall off of each individual surface portion. In this way, the Dotsko application provides an off-axis observer with a perceived image brightness which is constant, independent of changes in the viewing angle.
The present invention addresses the problem of rapid gain fall off as a function of viewing angle in a fundamentally different manner. Here, the emphasis is on re-examining the selection and combination of screen surface coatings so as to provide a front projection screen that exhibits very slow gain fall off as the viewing angle increases. The present invention takes advantage of the diffusing characteristics of a metallic, silver flake coating material, properly combined with a bonding material, to produce a front projection screen that provides the observer with constant image brightness over the whole screen surface.